Cardiovascular is important. Strength is also important. Large developed muscles looks good, but all you need it to have enough to take you through a fight.
Fitness is always important. Cardio endurance lets you keep going longer. Strength can help you out-muscle someone. However, taijutsu is not about being stronger. I used to do a lot of lifting. I found that I had a hard time relaxing and not relying on my strength to do things. I have since cut back on my lifting and have seen my ability to relax and just use my movements to do things improve. Now this may not apply to others. That is how it worked for me. The muscles do look good, and they can help you "cheat" to make something work. Heck, I could have just picked most people up and broken them in half, but that isn't what I'm trying to do with my taijutsu. I have yet to find a single thing in what we are studying that required the strength it took me to shoulder press 250 lbs, so, why continue to do it? Just my opinions.
on a serious note you shouldn't have to be strong inorder to do taijutsu. muscles tend to tighten you up, and in my experience its better to be flexible.
Strength on strength depletes your ability to fight. You will slug it out for ages until one of you tires out and then you may or may not have the advantage. To do taijutsu, you need to move. flexability is important, but so if moving your feet. If you wish to stand like a lemming waiting to get hit, the fine, you get hit and we shall see you in ER and laugh.
The more you practise, the more muscles you get. Not the big beefed up muscles that is expected from a K1 or UFC fighter, but the muscles that are there that make you move quick, give the strength needed to protect you. And with me doing this for 6 years now... yes 6 (regarding the negative comment about this thread of 'how long have you been doing this'), you get to find a way of moving unlike you did before you started learning.
But that is the physical. Other conditioning is the spiritual. And for that you need to develop as a person, not the red and white muscle groups in your body. Taijutsu is more than fighting, it is philosophy as well as physical. So develop both and become the better person than when you first stepped into the dojo.
